Further studies of factors affecting pathogenicity, including infectivity, further enquiry into pathogenicity mechanisms, and analyses of the relationships between these factors and mechanisms and the antigenic properties of trypanosomatids are to be conducted by cytologic and cytochemical (at the light- and electron-microscopic levels), immunologic, and biochemical methods. Mice, rats, hamsters, and rabbits, as well as vertebrate and invertebrate (tsetse) cell and/or organ cultures will be employed in the proposed research. (A) The following aspects of the african trypanosomes (T. brucei subspecies) are to be investigated: (a) the effect of various environmental factors on antigenic variations among bloodstream forms cultivated in vitro and on the production of metacyclics in an in vitro system and in vivo (Attempts will be made to cultivate T. congolense bloodstream forms in vitro and to restore infectivity in cultures of this species. Should these aims be achieved, experiments similar to those proposed for T. brucei subspecies will be conducted with T. congolense.); (b) the relevance of the culture transmission to cyclical in vivo transmission, primarily by antigenic analyses; (c) the immunogenic properties of the nonvariant antigens that appear to be limited to bloodstream forms; (d) the possible variant specific antigen heterogeneity on the surface of individual bloodstream trypanosomes, by the quantitative fluorescent antibody method; (e) antigenic analysis by the quantitative fluorescent antibody method of the transformation process from proventricular to metacyclic forms in smears of tsetse salivary gland contents. (B) With regard to leishmania spp., further chemical analysis and investigation of the effect of the antigenically active glycoproteins (formerly referred to as "excretory factors" (EF)) on the host's (mouse) immune response.